The purpose of a variable sector is to change the intensity of a light beam within the range from an arbitrary 100 percent to zero. For precise visual photometry it is necessary that the adjustment of intensity or brightness be made by simple means, thus permitting smooth and easy back-and-forth motions requiring almost no muscular effort and with little or no noise or other distortions. This variable sector is ideal in this respect. It was designed in 1907 by German physicist Eugen Brodhun, and this particular sector was purchased by the National Bureau of Standards from the firm Schmidt & Haensh prior to 1920. The sector is stationary and its aperture may be varied from the largest feasible opening down to cut-off on zero. The relative opening is precisely read on a linear scale and vernier to 0.1 percent between 110.0 percent and 0 percent. The light beam is carried rapidly across this sector aperture by means of a pair of rotating glass rhombs, the speed of rotation being great enough to eliminate flicker. This variable sector was installed in the Gibson photoelectric spectrophotometer and extensively used in that equipment until about 1950.

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